require referer field on registration

[jesus likes pie]

I have a forum which uses a product to make it invite only. When someone gets an invitation the mod automatically fills in the referer field so through a template edit I locked the field to make it unalterable.

Unfortunately I noticed that if there is an error during registration (like if passwords or emails don't match) when the page reloads to tell you there was an error it no longer has the referer field filled!

SO...is there a way to make registration fail if the referer field is empty? It is crucial that every user registers with a referer.

Thanks.